Previously Terraform would react to an invalid top-level command the same
way as for typing no command at all: just printing out the long top-level
help directory.

If someone's tried to type a command, it's more helpful to respond to that
request by explaining directly that the command is invalid, rather than
leaving the user to puzzle that out themselves by referring to the help
text.

As a bonus, this also allows us to use our "didyoumean" package to suggest
possible alternatives if it seems like the user made a typo.